CA1210516A

CA1210516A - Telemetry scheme

Info

Publication number: CA1210516A
Application number: CA000441585A
Authority: CA
Inventors: Houston B. Ii Mount
Original assignee: BP Corp North America Inc
Current assignee: BP Corp North America Inc
Priority date: 1982-12-22
Filing date: 1983-11-21
Publication date: 1986-08-26
Also published as: US4649550A

Abstract

ABSTRACT
A system for transmitting information contained in the form of bits. An encoded signal of two levels of magnitude is generated so that for each data word in the signal, the duration of the encoded signal at one level is equal to the duration of the encoded signal at the second level. The encoded signal is then used to generate a slope signal. It is this slope signal which is trans-mitted.

Description

12~S~ 8513 Mount TELEMETRY S~HEME
BACKGROUND OF THE INVENTION
-This invention concerns a method of transmitting information contained in bits. It particularly pertains to the transmission of information between an instrument 15 lowered in a borehole drilled in the earth and the sur-face. It is especially adapted to wireline well logging which is accomplished by suspending specialized devices in a well on a long cable and measuring various properties within the well in which it is drilled and transmitting 20 the information to the surface over the wireline.
As it is well known, wells are drilled deep into the earth in the search for and the production of oil and gas. It is important to perform many different operations downhole with specialized d~vices lowered on a wireline.
25 It is also important to be able to control the specialized device from the surface and also of even greater impor-tance is to transmit information detected ~y the device to the surface. A typical cabl~ used for well logging is seven No. 20 gauge insulated conductors, spiral wrapped in 30 two layers of steel wire, and may be as long as 30,000 ft.
In the existing system~ more then a microfarad of capacity may exist between any two conductors or between any single conductor and the steel sheath surrounding the conductors.
If more than two or three measurements or con-35 trol functions or channels are to be carried out throughthe seven existing conductors in the example given, some method of timesharing must be used. One con~on method of doing so is to digiti~e the signals and transmit the ~t~

121(~16

-2-information one binary digit or bit at a time, one channel after another. Any number of channels can be transmitted along a signal pair of wires in this fashion limited by the frequency bandwidth of the cable and the maximum S amount of time that the user considers acceptable to cycle through all channels. Special transmission of binary digital data can ordinarily be accomplished by repre-senting a binary one as one selected DC voltage signal, for example five volts and a binary zero as another DC
10 voltage, for example, zero voltage. Digital data trans-mission in this manner might be observed on an oscillo-scope. Due to the length and capacity of a logging cable, however, transmission of ~C voltages in this fashion is somewhat impractical. The bits of information become dis-15 torted as data rates increase until the signal cannot bedecoded. This is primarily a result of the limited cur-rent capacity of the transmitter not being able to drive the wireline to the appropriate voltage quickly enough and the limited bandwidth of the cable. A common method to 20 avoid this problem is by the use of frequency modulation.
The ones and zeros of the binary data are coded as bursts of two distinct frequencies. For example, the ones might be coded for 300 Hz and the zeros for 600 Hz. These sig-nals can be detected and separated at the receiver 25 throughout a wide range of amplitude, and the data can then be reconstructed.
SUMMARY OF THE INVENTION
This invention concerns a method of transmitting information contained in serial form in bits. In a pre-30 ferred form, I obtain an encoded signal by generating aset of two segments for each such bit. In each said set one segment is of a voltage level different from the other. A slope modulated signal is then generated from the encoded signal and is transmitted as a slope modulated 35 signal. An example of a slope modulated signal is one with a rising slope during one voltage level of the encoded signal and a falling slope during the other vol-tage level of such encoded signal. Means are provided at ~6 ~he receiving end of the signal transmission system to convert the transmitted signal into the original signal or what other form may be desired, It is thus an object of this invention to 5 describe a new and more efficient manner of transmitting large amounts of data over wirelines.
BRIEF D~SCRIPTION OF THE DRAWINGS
.
Figure 1 illustrates a wireline tool suspended in a wellbore.
Figure 2 is a typical format of a serial data word.
Figure 3 sho~s Signal A which is a square wave signal; Signal ~ which is Signal A in frequency modulated form; Signal C which is Signal A encoded to be slope modu-15 lated; and Signal D, which is a slope modulated signal.
Figure 4 is a functional diagram of the trans-mitter portion of the system.
Figure 5 is a block diagram of the surface analog processing hardware.
Figures 6, 7, 8~ and 9 depict a flow diagram for software for use in developing the slope signal of this invention.
Figure 10 illustrates a circuit for controlling the slope of the slope signal.
DETAILED DESCRIPTION OF THE P~EFERRED EMBODIME~T
_ Attention is first directed to Figure 1 which illustrates a specialized device 10 such as a logging tool or sidewall core cutter in a wellbore 12 suspended by line 14 over a counter pulley 16 at the surface. Cable 14 30 extends through 2 power drum 18 to a surface recorder 20.
It is in this setting that I anticipate the largest use of my invention. The wellbore 12 may have a depth of any-where from a few hundred feet deep to 15 to 20 thousand feet or more. The counter wheel 16 is typically used to 35 determine the amount of cable 14 which is lowered into the wellbore so that the position of tool 10 can always be accurately determined. Drum 18 is a typical drum which is commercially used by many well service companies and pro-12105~;
--4--vides the power to tool 10 and also provides means for raising and lowering cable 14. The cable 14 is connected through cable 19 to recorder 20 which records the signals from tool 10. If desired, a transmitter 22 can be 5 connected through cable 21 to cable 14 and signals can be transmitted down the cable to tool 10. It is this setting of Figure 1 in which I anticipate the largest use of my invention will be made.
A typical cable used for well logging is seven 10 No. 20 gauge insulated conductors, spiral wrapped in two layers of steel wire, and may be as long as 30,000 ft.
Within a microfarad of capacity may exist between any two conductors or between any single conductor and the steel sheath surrounding the seven conductors. If more than two 15 or three measurements or control ~unctions or channels are to be carried out through the existing conductors, some method of timesharing must be used. One common method of doing so is to digitize the signals and transmit the information one binary bit or digit at a time, one channel 20 after another. Any number of channels can be transmitted along a single pair of wires in this fashion, but it is limited by the frequency bandwidth of the cable and the maximum amount of time that the user considers acceptable to cycle through all channels. A typical format of a 25 serial data word is shown in Figure 2. Shown there is a word indicated to have a total of 45 bits. It has a start section at 24 and a stop section at 26. The start section 24 has two bits, one of a zero voltage or representing zero, and the other level representing a one which may be 30 one or five voltage or whatever might be desired.
After the two start bits, there are 4 bits which are used here to identify the number of the data channel which is being transmitted in this serial data word. Here the first bit (that. nearest start bits) is used to repre-35 sent the least significant ("ones place") bit ~LSB) of thebinary channel number. The fourth bit to the right of the start bits represents the most significant bit (MSB) ("8's place") of the channel number.

The twelve bits following the channel number are the data bits associa-ted with the channel number. They are arranged here also least signi~icant digit on the left (i.e., first in time) in ascending order to the most sig-5 nificant bit on the right.
Signal A of Figure 3 shows a square wave signal having a zero value and a plus five voltage for a one value. The value of these voltages can be any convenient value so long as they are adequately separated. Signal A
10 represents a signal containing the information which you desire to transmit. However, transmission of DC voltages in this fashion is somewhat impractical. The bits of information become distorted as data rates increase until the signal cannot be decoded. This is primarily a result 15 of the limited current capacity of the transmitter not being able to drive the wireline to the appropriate vol-tage level quickly enough and of the cable bandwidth. A
common method used to avoid this problem is "frequency modulation". The ones and ~eros of the binary data are 20 coded as t~o distinct frequencies. For example, the ones would have 300 Hz and the zeros have 600 Hz. This is indicated in Figure 3, Signal B, in which the logical one is represented by a higher frequency than is the logical zero of Signal A. Ihe method which I propose in this 25 invention involves the use of slope modulation. In the slope modulation method~ a binary one is represented by a rising voltage or a positive slope and the binary zero as a falling voltage or a negative slope. The absolute vol-tage of the telemetry signal is not important, only 30 whether it is rising or falling. Signal C of Figure 3 shows a serial binary string derived from digital data shown in Signal A. In this case, Signal C is a balanced encoded telemetry signal and can be obtained by Manchester type encoding. As can be seen, each bit of Signal A is 35 divided into a logical one and a logical zero bit. Each segment of each bit representation of Curve C is of a dif-ferent voltage level. Segment 30 is positive because bit 29 which it represents is positive. Then, segment 32 is the opposite polarity or lower voltage from that of segment 30. On the other hand, segment 34 is the lower voltage because the bit 31 which it corresponds to is the lower voltage. The second segment 36 representing bit 31 5 is of the higher voltage. Thus, we obtain an encoded balanced telemetry Signal C from the normal digital data of Signal A of ~igure 3.
Attention is next directed to Signal D of Figure 3. In this case, I have converted the Signal C to 10 a slope modulated signal; a positive value 30 as a posi-tive slope 30A and the lower value 32 as the negative slope 32A. In other words, high levels have been con-verted to positive slopes and low levels to negative slopes. Note tha~ either a positive or negative slope can 15 exist at al~y absolute voltage. It is important to note that the maximum voltage and the minimum voltage obtained in Curve D is always within a controlled limit. Thus, the raw data of Signal A in Figure 3 has been encoded in a manner such that the average time that the slope modulated 20 signal is rising equals the average time that it is falling. What is done is representing each binary bit of data of Signal A as two sequential bits at opposite states. This prevents the signal from reaching voltage limits imposed by the transmitter. It is normal that a 25 transmitting device has some finite limit on its voltage range, perhaps plus or minus 15 volts.
Advantages of the slope modulation method described herein as compared to the frequency modulated method of Signal B are simplicity and speed. Hardware 30 implementation of transmitters and receivers is simpler than for frequency modulation and a large range of trans-mission rates can be used with no change in hardware.
This has very simple adaptation to various transmission line characteristics. Similar adaptation with frequency 35 modulating equipment generally requires changes in modula-tion fre~uency and associated hardware in order to main-tain optimum data rates. Regarding speed, the ma~imum data rate using slope modulation is at least several times ~2~5i6 that possible with frequency modulation ~or a given transmission bandwidth. This is due to the fact -that at least several cycles at a given frequency are required in a frequency modula~ed system in order to reliably recog-5 nize the data bit as a one or a zero. On the other hand,using the method described herein, only one cycle is required in slope modulation to recognize a bit.
Attention is next directed to Figure 4 which is a functional outline or diagram of the transmitter portion 10 of my system which would be use, for example, with special device 10 of Figure 1 to obtain the slope Curve D of ~igure 3. Block 40 represents the encoded telemetry system from which Signal C of Figure 3 is fed to the transmission portion. This transmission portion includes 15 a line 44 connected to a line driver 50. The line driver accomplishes DC level shifting and current amplification resulting in a signal that is capable of driving the high capacity well logging cable. The output of line driver 50 is connected through a resistor 54 to one conductor 58 of 20 the cable 14 of Figure 1. The output of line driver 50 is fed through resistor 54 which is identical to resistor 52.
The output of resistor 54 is connected to conductor 28 of cable 14 of Figure 2. The resistors 52 and 54 are selected to limit the maximum current drawn from the line 25 drivers 50 and 48 and also to control the rise and fall time o~ the resulting slope modulated signal. In this arrangement the slope will not be a straight line but actually logarithmic due to the RC circuits formed by the cable capacity and resistors 52 and 54. If the line dri-30 vers are current limited devices, resistors 22 and 24 canbe deleted and a straight line slope can be generated instead of the logarithmic function. The slope can then be controlled by adding capacity to the OUtpllt of the line driver as shown in Fig. 10 which shows parallel line dri-35 vers 80 and 8~ started by capacitor 84. Line driver S0 isprovided with an inverter 86. Item 46 is a computer logic inverter and may be a part of a Texas Instruments circuit designated SN 7404. Items 48 and 50 are each sections of 5~6 an IEEE designated RS-232 line driver device, which may be a National Semiconductor ~S 1488. Items 52 and 54 are common 1/4 or 1/2 watt resistors.
In the device of Figure 4, encoded telemetry 5 Signal C from system 40 is fed through a branch line 42 and inverter 46 to line driver device 52. An inverted signal is generated by inverter 46 and drives the other wire 56 of the signal. This part of the circuit functions identically to the line driver resistor circuit 50 and 10 resistor 54 previously described, except the encoded tel-emetry signal from block 40 is first inverted by logic inverter 46 before being slope modulated by the line driver 48 and resistor 22. This additional driver circuit doubles the differential signal voltage received at the 15 other end of the wireline. Inverter 46, line driver 48, and resistor 52 can be eliminated in the wireline 56 con-nected directly to the telemetry transmitting power supply common if the additional voltag~ swing is not needed.
Figure 5 is a functional diagram of the receiver 20 portion of the system and is located at the surface. It includes an instrument amplifier 59, a differentiator 71 and zero crossing detector 73. Instrument amplifier 59 is connected to lines 56 and 58 and converts the differential signal from the wireline pair to a single ended signal 25 indicated by Signal 31, i.e. a signal which is ground ref-erenced. The amplifier 60 includes resistors 39 and 41 which serve to terminate the line and provide bias current for the amplifier 60. The resulting single ended signal, indicated by Signal 31, can be observed at the output of 30 the amplifier 59. This signal is then further processed through a differentiator stage 71 indicated by dotted line which converts the rising slopes into negative voltages and falling slopes into positive voltages as indicated at signal 73. Differentiator circuit includes a capacitor 64 35 which has an output divided into lines 66 and 68. Line 66 goes to resistor 70 and line 6~ goes to operational ampli-fier 67. Thus, the differentiator stage 45 comprises an operational amplifier 67, a resistor 70 and a capacitor 5~
g 64. The resistor/capacitor combination is selected to optimize the differentiation function for a given signal frequency. The output from this stage is indicated by signal 73. The output signal 73 is then processed by ~ero 5 crossing detector indicated by block 75. ~ero crossing detectors are well known and may comprise a voltage compa-rator 74 and may include a resistor 76. The output from zero crossing detector 72 is indicated by signal 78. It is thus seen that the zero crossing detector 72 serves to 10 improve the shape of the signal pulse 73 and to shift the voltage levels to those appropriate for logic electronics as indicated at signal 78.
The encoded signal which has been reconstructed by the receiver as illustrated in Figure 5 can be decoded 15 by various hardware logic or programmable logic. Commer-cial devices are available for encoding and decoding binary digital data in Manchester code, if the flexibility of a programmable system is not required. A Harris HD-6402 universal asynchronous receiver/transmitter with a 20 Harris HD-6408 asynchronous serial Manchester adapted can be used at each end of the system for data work linked up to 8 bits. These receiver/transmitters and adapter are commercially available from Harris Corporation. More com-plex data formats can best be accommodated by using the 25 programmable computer hardware. In this regard attention is next directed to Figures 6, 7, 8 and 9.
Attention is first directed to Block 102 of Figure 6 which represents the starting position of the flow charts of Figures 6 to 9. The main transmit program, 30 indicated by blocks 104, 106, 108, 110, 112 and 114, is entered from some other operating system program. The operating system program is simply any system which will pass the data which is to be transmitted to the surface.
It will preferably be passed through some specific loca-35 tions in memory. Time allotted to transmit each bit isfixed at a value selected to meet data transmission rate requirements without exceeding the bandwidth of the trans-mission line. The data to be transmitted is in the form 12~0Sl~i of a dataword J times N bits in length. An example would be a dataword 20 bits long to be broken into five seg-ments, each four bits in length. In this example~ J would equal five and N would equal four.
Information to be transmitted is fed to Block 104 which is a memory location called JCOUNT which is initialized to zero so that it may be used as a counter to index which segment o~ N bits is to be transmitte~.
A synchronization sequence, indicated by 10 Block 106, is transmitted at the beginning of each data-word. This sequence is chosen to be identifiable ~rom any possible sequence of data which might be transmitted and is used by the data receiving system to recognize the start of a dataword and to lock into the timing of the 15 sequence.
In Block 108 one of the N bit sequences of data is selected according to the index counter JCOUNT. In Block 110 the selected N bit sequence is passed to the transmit subroutine for transmissic,n and the transmit 20 subrou$ine program is executed. Control is returned to Block 112 in the main transmit program and JCOUNT is incremented to point to the next N bit sequence to be transmitted. Block 114 is a decision block and determines if all J segments of the dataword have been transmitted.
25 If so, control is returned to the operating system; if not, the program loops back to Block 108 to transmit the next segment. Block 116 is labeled the "Exit" and is used to tie the components of the flow chart of Figure 6 to operating system program shown in Figure 7.
Attention is next directed to Figure 7.
Block 118 is labeled "START" where one N bit data sequence is passed to the transmit subroutine as control is trans-ferred from the main transmit program of Figure 6.
In Block 120 a memory location called NCOU~'T is 35 preset to zero to be used to index which bit of the N bit data segment is to be transmitted. In Block 122 a loca-tion called COUNTl is used to count the number of 'binary "ones" contained in the N bit segment. In Block 124 ~%~5~

COUNT~ is used to count "zeroes" in the N bit se~ment. In Block 126 one bit of the N bit segment is selected according to the bit index counter, NCOUNT.
Block 128 is a decision block to determine 5 whether that bit is a "zero" or a "one". Control goes to Block 130 if the data bit is a one, or to Block 134 if it is a zero.
Block 130 labeled COUNT1 is incremented if the data bit ~as found to be a "one". The telemetry output 10 line is commanded to transmit a "mark" or a one for one bit time period by Block 132.
In Block 134, COUNTZ is incremented if the data bit was found to b~ zero.
In Block 136 the telemetry output line is com-15 manded to transmit a "space" or a zero for a single bittime period.
NCO~NT is incremented in Block 138 whether the data bit is a one or a zero.
In decision Block 140 NCOUNT is checked to det-20 ermine if the entire ~ bit se~uence has been transmitted.If not completely transmitted, the program loops to Block 126 to transmit the next bit.
After all N bits are transmitted, Block 142 det-ermines the required type of correction bits required. If 25 the number of binary ones transmitted was greater than the number of zeroes, control passes to Block 144; otherwise to Block 148.
Block 144 determines the number of correction "spaces" to be transmitted.
ln Block 146 the appropriate number of "space"
bit time periods is commanded at the telemetry output 1 ine .
In Block 148 the number of correction "marks" to be transmitted is determined. The appropriate number of 35 "mark" bit time periods is commanded by Block 150.
Block 152 is used to return control to the main transmit program.

l~lV5~6 The receive main program will, when executed, synchronize on a transmitted sequence of data and decode the J times N bits of the dataword. The dataword and pro-gram control will then be passed back to the operating 5 system program until more data is expected. The main receive program starts at Figure 8, Block 160. In Block 162 a memory location called JRECEV is initializ~d to be used as a counter to index which segment of N bits of data is being received. Block 164 identifies the 10 selected synchronization sequence and locks the receive program to the timing of the rest of the dataword. The receive main program transfers control to the receive subroutine for reception of an N bit sequence of data in Block 166.
When control is passed back to the receive main program along with the last N bit data sequence, that sequence is stored according to the index counter, JRECEV, Block 168. JRECEV, Block 170, is incremented to index the next sequence of N bits. Decision Block 172 determines if 20 all J segments of data have been received. If not, the program loops back to Block 166 for the next segment.
Control is passed to this Block 174 when all J segments of data have been received and the entire dataword has been reconstructed. Control is then passed back to the oper-25 ating system program which will make use of the data.
Figure 9 shows the telemetry Receiver SubroutineProgram which can reconstruct the original transmit signal.
The receive subroutine program is entered from 30 the receive main program at start Block 180 and is already in synchronism with the dataword so that the next N bit sequence is just starting. A memory location Block 182 called NRECEV is preset to zero to be used as a counter to index which of the N bits of the data segment is being 35 received. A memory location, Block 84, called RCVZ is preset to zero to be used to count the number of binary "zeroes" received in the N bit data segment. Another location, Block 186, called RCVl is also preset to zero to count the number of binary "ones" received in the data segment. In Block 188 one data bit is input based on the fixed time allowed for transmission of a single bit. The bit is also saved in a location indexed by counLer NRECEV
5 indicated at Block 196 for reconstruction of the whole N
bit data segment.
Decision Block 190 determines if the data bit received was a binary "one" or a "zero". If it was a "one", control goes to Block 192; otherwise to Block 194.
RCV1 counter 192 is incremented if the received data bit was a binary "one". RCV2 counter 194 is incre-mented if data was a "zero". NRECEV index counter 196 incremented. NRECEV 198 is checked to see if all N bits have yet been received. If not, the program loops to 15 Block 188 to input the next bit.
In Block 200 the difference between the number of "one" and "zero" bits received is calculated and a delay of that number of bit time periods is generated in order to maintain synchronization with the next N bit data 20 segment. The reconstituted signal exits through Block 202 and control is returned to the receive main program.
While the above description has been made in detail, various modifications can be made thereto without departing from the spirit or scope of the invention.

Claims

WHAT IS CLAIMED IS:

1. A method of transmitting information con-tained in bits which comprises the steps of:
(a) generating a set of two segments for each said bit and in each said set one segment is of a magnitude different from that of the other to obtain an encoded signal;
(b) generating a slope modulated signal from said encoded signal of step (a);
(c) transmitting said slope modulated signal.

2. A method of transmitting information con-tained in bits which comprises the steps of:
(a) generating data words of n bits;
(b) adding m correction bits as necessary to obtain modified data words in which each data word has a number of bits at one magnitude generally equal to the number of bits at a second magnitude;
(c) generating a slope modulated signal from said modified data words and;
(d) transmitting said slope modulated signal.

3. A method as defined in Claim 1 including receiving and processing the transmitted slope modulated signal.

4. A method as defined in Claim 2 in which the processing step includes digitizing the slope modulated signal.

5. A method as defined in Claim 2 in which the processing step includes converting the received slope modulated signal to the same form as the data word of step (a) of Claim 2.

6. A method as defined in Claim 2 in which in step (b) each data word has number of bits at one magni-tude exactly equal to the number of bits at the second magnitude.

7. A system of transmitting information con-tained in bits which comprises:

(a) first means for generating an encoded signal having a set of two segments for each said bit, one segment in each said set having a different magnitude from the other;
(b) second means connected to the output of said generating means for generating a slope modu-lated signal from said encoded signal;
(c) means connected to said second means for transmitting said slope modulated signal.

8. An apparatus for generating information contained in bits which comprises:
(a) first generating means to generate data words of n bits;
(b) adding means to add m correction bits to obtain modified data words in which each data word has a number of bits at one magnitude essentially equal to the number of bits at a second magnitude;
(c) slope generating means to generate a slope modulated signal from the modified data words of said adding means;
(d) transmitting means for transmitting said slope modulated signal from said second gener-ating means.

9. An apparatus as defined in Claim 8 in which said slope generating means includes a line driver and a resistor in series.

10. An apparatus as defined in Claim 8 including an instrument amplifier connected to the trans-mitting means, a differentiator circuit connected to the output of said instrument amplifier to obtain a square wave output signal and a zero crossing detector connected to the output square wave signal of said instrument ampli-fier.